The long term goal of the proposed research is to elucidate the biosynthesis by 5-epimerization of L-iduronic acid in mammalian mucopolysaccharides heparin and dermatan sulfate and of L-guluronic acid in the alignate of various bacteria. Approaches will include examination of the interconversion of UDP-D-glucuronic acid and UDP-L-iduronic acid, determination of the fate of H3 atoms of saccharide precursors upon conversion of the latter to polymers, and purification of the 5-epimerases which catalyze the conversion of the D- to L-uronic acids, both at the sugar nucleotide and at the "in polymer" level.